1. Field of the Invention
This invention relates to a CVD apparatus for forming a film by using a hot element set at a prescribed temperature and a method for removing a deposited film, and particularly to a hot element CVD apparatus in which an in-situ cleaning is applied to and its cleaning method.
2. Description of the Invention
In the manufacturing process of semiconductor devices such as an LSI (large scale integrated circuit), display devices such as an LCD (liquid crystal display) and the like, a chemical vapor deposition (CVD) method is widely used for forming a variety of thin films on a substrate.
As such a CVD method, a plasma CVD method which utilizes a plasma to decompose and/or activate a material gas to form a film on a substrate and a thermal CVD method which utilizes the heat of a substrate to cause a reaction of a material gas and form a film on a substrate are well know. In addition to these methods, there is another CVD method in which a material gas is decomposed and/or activated to form a film by a hot element set at a prescribed high temperature. This CVD method is called “a hot element CVD method” hereinafter.
A CVD apparatus to carry out this method is constructed so that a hot element made of a metal having a high melting point such as tungsten and a substrate are disposed in a processing vacuum chamber, and a material gas is introduced into the chamber while the hot element is maintained at a temperature of about 1000–1800° C. The material gas introduced in the chamber is decomposed and/or activated to generate activated species when passing over the surface of the hot element. The activated species reaches the substrate and forms a thin film on the surface of the substrate. Of such a hot element CVD method, one using a wire as a hot element is referred to as a Hot-Wire CVD method, and one that is thought to be utilizing the catalytic reaction of a hot element in decomposition or activation process of the material gas is referred to as a Catalytic-CVD (or cat-CVD) method.
In the hot element CVD method, the decomposition or activation reaction of the material gas occurs when the gas passes over the surface of the hot element. Therefore, the substrate temperature can be lowered as compared with a thermal CVD method in which such a reaction is caused to occur only by the heat of the substrate. And unlike a plasma CVD method, the substrate is free from the damages caused by plasma. For these reasons, the hot element CVD method is expected to be a promising film forming method of semiconductor devices and display devices of the next generation, which will have higher integration and higher functions.
As the film formation is made on a substrate, a film is also deposited on the inner structures of the film forming apparatus in any film forming method including CVD methods mentioned above. The film deposited on the inner structures peels off when it becomes thick, which produces particles. The particles thus produced may be incorporated in the film on the substrate or adhere to the surface of the film, which causes the defect in the device and reduces production yield of the device.
In order to avoid these problems, the deposited film on the inner structures should be periodically removed before peeling off in the repetition of film formation.
One method often used for preventing the generation of particles due to the peeling-off of the deposited film is to cover the inner surfaces of the apparatus with sheets or members on which the films can be deposited instead of on inner surfaces in the apparatus, and to periodically exchange these sheets or members. However, since the deposition takes place even inside the narrow gaps of inner structure or on the hidden side of the sheets or members in the case of, e.g., the CVD apparatus. It is impossible to completely prevent the generation of particles.
In contrast, as a method for removing a film deposited inside the film forming chamber, there is a method called an in-situ cleaning method in which a cleaning gas is introduced into the chamber and is made to react with deposited films with the aid of energy of plasma or heat to generate gaseous substances which can be exhausted.
Since the in-situ cleaning method is carried out without exposing the inside of the chamber to the atmosphere, the continuous and stable production of thin films having a prescribed characteristic becomes available. In addition, since neither the exchange of the sheets or members nor the operation of exhausting the chamber from the atmospheric pressure to a prescribed pressure is necessary, the time for the cleaning process is remarkably shortened. Thus, this method has an advantage in increasing the productivity.
Moreover, this method makes it possible to remove even the film deposited inside the narrow gaps and effectively suppress the generation of particles.
When the in-situ cleaning method is applied to a plasma CVD apparatus used for forming, for example, a silicon film or a silicon nitride film, a cleaning gas such as NF3, CF4 or CCl4 is introduced into the processing chamber and the plasma is generated at a proper time after the film formation has been repeatedly carried out. The cleaning gas is decomposed and/or activated by the plasma to react with a deposited film. A silicon film is converted to silicon tetrafluoride (SiF4) or silicon tetachloride (SiCl4) and a silicon nitride film is to SiF4 or SiCl4 and nitrogen (N2). Since these gaseous substances are to be exhausted, the deposited film, consequently, can be removed.
In contrast, when a cleaning gas such as ClF3 that is easily decomposed by heat is employed, plasma is not necessary. The deposited film can also be converted to gaseous substances merely by heating the chamber. However, the chamber must be heated above 200° C. in order to obtain a practical removal rate, which may cause the degradation of vacuum seals and substantially lengthens the cleaning time because it takes long time to heat and cool the chamber.
As has been mentioned, the in-situ cleaning of the film forming chamber is a very important process that is used to stabilize and continuously form a film with prescribed characteristics. Therefore, the present inventor attempted to apply the in-situ cleaning method to a promising hot element CVD apparatus and found that the convention in-situ cleaning methods have a problem in that a hot element made of metal having a high melting point reacts with the cleaning gas, and the wire becomes thin. That is, in this attempt, there were placed electrodes for the plasma generation in the processing chamber, the cleaning gas was introduced and then plasma is generated to carry out the cleaning. Although the deposited film could be removed, the hot element was also etched so that its diameter decreases. As a result, the prescribed exothermic characteristics could not be obtained in the next film formation.
A ClF3 gas as a cleaning gas was introduced into the film forming chamber which was heated to 200° C. by using a heater installed outside. The hot element similarly reacted with the cleaning gas and its diameter decreased.
Thus, conventional in-situ cleaning methods cannot be directly applied to a hot element CVD apparatus. However, since in-situ cleaning is desired for the continuous and stable production of films, further examination on the structure of the hot element CVD apparatus and the cleaning condition were made to establish the in-situ cleaning technology is applicable to the hot element CVD apparatus.